German Patent Aplication No. 38 33 212 (corresponding to U.S. Pat. No. 5,272,634) describes, for a vehicle driven at a single axle and equipped with an antilock braking control system, the creation of a reference variable for the vehicle speed outside the braking pressure control system. This is done essentially by determining an averaged wheel speed on the basis of the speeds of the two driven wheels. If the derivative over time of this averaged wheel speed is greater than zero, the reference variable is defined by the speed of the slower of the two driven wheels. If the derivative over time of this averaged wheel speed is less than zero, the reference variable is then defined by the speed of the faster of the two driven wheels. The slope of the reference variable for the vehicle speed is thus limited to a maximum value.
German Patent Application No. 40 09 195 (corresponding to U.S. Pat. No. 5,364,174) describes, for an antilock braking control system used in an all-wheel-drive vehicle, the creation of a reference speed required for traction control. In this context, the reference speed is essentially defined by the speed of the slowest wheel in the case of a speed increase, and by the speed of the third-fastest wheel in the case of a speed decrease. In the transition region between increase and decrease, the value attained by the slowest wheel speed is held constant. For the case where the speed of the third-fastest wheel deviates too greatly from an auxiliary reference speed, the reference speed is maintained concurrently with that auxiliary reference speed. The auxiliary reference speed is created by raising it by the speed of the third-fastest wheel, and decreasing it by the speed of the fastest wheel. It also is held constant in a transition region. If an instability exists due to incipient locking, the slope of the reference speed is defined by the slope of the auxiliary reference speed. If the wheels are slipping, the reference speed is held constant and the rise in the auxiliary reference speed is limited to a physically possible vehicle acceleration. If the rear wheels of the all-wheel-drive vehicle are decoupled from the drive system during braking, a slightly modified approach to creating the reference speed is used. Outside the control system, the reference speed is raised if the speeds of all the wheels are greater than it. If two or three wheels are faster than the reference speed, it is held constant. If none or only one of the wheels is faster than the reference speed, the reference speed is set downward.
Systems for controlling brake slip or traction are known in general form, for example from the book "Bremsanlagen fur Kraftfahrzeuge" (Braking Systems for Motor Vehicles) published by Robert Bosch GmbH Stuttgart (VDI-Verlag, Dusseldorf, 1st edition 1994).
An object of the present invention is to improve the manner in which a variable describing the speed of a vehicle is determined.